This invention relates to specialized valves for use in pressurized fluid systems. More specifically, it relates to liquid marking systems wherein ink is applied in droplets or a stream of material to various surfaces for marking purposes. Such systems are often referred to as drop on demand or ink jet marking or printing systems. Typically, ink is pressurized and forced through a nozzle to create tiny drops which may be controlled electrostatically or otherwise to determine where and when they will strike the article to be marked. Drop on demand systems create and project drops toward the article to be marked only when marking is to be done.
For brevity throughout the specification such systems, regardless of type, will be referred to as ink jet systems. Ink jet printing equipment is used for many industrial marking applications as, for example, date coding on food and beverage packing lines, addressing magazine labels, and the like. An important requirement of such systems is that the print head, containing the ink nozzle or orifice which forms the droplets or a stream of marking material, be located remotely from the ink supply and control electronics. Thus, the print head is usually supplied with ink from a centralized ink supply via a flexible conduit of some length, on the order of ten to thirty feet.
An effective ink jet system must be able to control (turn on and off) the supply of ink to the nozzle rapidly and accurately. In addition, the control or shut off device, usually a valve, must prevent two phenomena which adversely affect print quality. The first is referred to as "drool" wherein during a nonprinting period the depressurized ink supply line is not effectively shut off, permitting the nozzle to drip resulting in unacceptable printing when the nozzle resumes operation. The second phenomenon is the reverse of the first. Back pressure can cause entry of air into the nozzle. This too results in unacceptable operation including missed information.
In the prior art several approaches to controlling the flow of ink to the nozzle have been employed. One such prior approach (see U.S. Pat. No. 4,067,020) is a pneumatically controlled valve located close to the nozzle. This valve, controlled by a separate source of air pressure, provides acceptable ink flow gating but, due to its size and complexity, interferes with the desire to minimize the dimensions of the print head.
A second prior art approach is to locate the ink valve remotely from the print head. With this approach, however, it is necessary to take into account the pressure build up in the walls of the flexible conduit connecting the ink supply to the nozzle. When the ink flow is stopped the energy stored in the flexible conduit walls must be dissipated or drool will occur contaminating the print head, particularly in the case of electrostatic systems. To deal with the energy stored in the conduit walls the prior art has employed a three way valve which, when ink flow is stopped, has a vent port through which the pressure in the conduit walls is dissipated by permitting ink to pass therethrough. This approach is not entirely satisfactory because the valve position relative to the nozzle must remain fixed. If not, shut off cycles will be slow and drool and/or suction will occur.
A third prior technique (see U.S. Pat. No. 4,234,885), locates the ink valve at the ink supply rather than adjacent the print head. To prevent the storage of energy in the flexible conduit walls a pressure jacket is provided surrounding the conduit. This system is somewhat complex and expensive.
All of the foregoing prior art techniques, in addition to the drawbacks mentioned, must be provided with a separate control medium, such as electric or pneumatic control.
It is desirable to provide an improved valve system for controlling a supply of ink to a print head which overcomes these disadvantages. More specifically, it is an object of the invention to provide a small, high speed valve which can be mounted near, preferably adjacent, the ink jet nozzle. Such a design obviates the need for a pressure jacket surrounding the ink conduit and permits a reduction in the size of the print head facilitating use of the ink jet marking system in many additional applications. It is further desirable to develop a valve which does not require a separate control medium.
It is accordingly an object of the present invention to provide such a small, simple valve which is low in cost but has a high performance level.
It is a further object of the invention to provide a valve which can be mounted near the ink jet nozzle and which does not require a separate control medium but rather operates as a function of pressure in the ink supply conduit.
Another object of the invention is to provide an ink valve which has a high cracking pressure to prevent drool and which has a negligible pressure drop across the valve.
Other objects and advantages of the invention will be apparent from the remaining portion of the specification.